Aircraft generator systems have typically been adversely impacted by the speed range of the engines. As a consequence, sophisticated and complex speed control methods have been adopted to maintain the generator at a constant speed over the speed range of the engines, or complex electronic power converters have been used to convert the variable frequency power output of the generator to constant frequency power.
The trend toward "all-electric" airplanes and "all-electric" environmental control systems will most probably necessitate the selection of generators of large capacity; e.g., 250 to 500 kva generators. As a consequence, it would be very penalizing for a conventional type constant speed drive, or a variable speed constant frequency system, to maintain constant output power over the full speed-range of the engines. Aside from the torque and current sizing problems that impact on weight and size, there is the question of fuel-efficiency, which is impacted by the overall transmission efficiency. On the other hand, as has been described in co-pending U.S. patent application Ser. No. 173,111, filed July 28, 1980, for "A Direct-Driven Aircraft Generating System Providing Variable and Constant Levels of Electric Power", assigned to the assignee herein, the majority of the loads in a typical large aircraft can be powered by a generator system, whose voltage and power is proportional to frequency (engine speed).
In addition, it would be highly desirable to utilize squirrel-cage rotor induction generators, which are extremely rugged and reliable machines, to provide power to the majority of loads. Such generators use relatively inexpensive materials in the rotor, and they can normally be easily fabricated. There are, however, also certain problems associated with induction generators. First, to operate as a generator, the rotor must be driven above its synchronous speed; i.e., in a negative-slip condition. Second, the machine cannot operate as a generator unless there is at least one synchronous generator available in the system to excite it. In addition, the machine cannot supply either its own excitation or (lagging) reactive kilovars.
It is a primary object then, of the present invention, to provide a drive mechanism of minimum weight and complexity, which will control the excitation frequency of an induction-machine and cause it to operate in a generating mode.
It is a further object of the present invention, to provide a multi-generator aircraft power generation system that includes an induction-machine for providing primary power, an exciter machine, and a differential variable-speed drive that controls the negative slip frequency between machines as a function of the input speed and the electric load on the induction machine.
It is yet another object of the present invention to provide a power generation system, particularly for aircraft, that utilizes an extremely reliable induction machine as a power generator within the system.
It is a further object of the present invention, to develop a novel drive and a multi-machine power system which utilizes at least one minor synchronous machine in conjunction with a primary induction machine.
These and other objects of the present invention will become more readily apparent after a full consideration of the following description of the instant invention, and the several advantages thereof enumerated herein.
A variety of differential and variable-rate transmission and drives utilized in conjunction with electric machines are known in the prior art. Such drives are typified by those disclosed in U.S. Pat. Nos. 2,810,844 to Morrill; 3,032,696 to Payne et al; 2,077,768 to Perry; 2,990,503 to Clark; and 2,153,252 to Hunsdorf.
In the U.S. Pat. No. 2,810,844 patent, a constant speed drive type system is disclosed which uses a differential planetary gear arrangement to control the frequency of a dynamoelectric machine and to derive a modified selected frequency different from the base frequency of the machine, while the U.S. Pat. No. 3,032,696 patent discloses an engine driven generator/motor that includes a variable-rate transmission interposed between the engine and generator. The drive system includes bi-directional (overrun) clutches to prevent direct drive of the transmission in the generator mode, and gear-reduction in the motor (start) mode. In addition, a current sensing control (current-transformer) is used to monitor and control the power taken by the motor/generator.
A synchronous drive system is disclosed in the U.S. Pat. No. 2,077,768 patent, wherein a frequency changer is driven at a variable speed and rotary induction machines are electrically "locked in step" with the frequency generator. In this particular system, the machines operate at the speed of the frequency generator. The U.S. Pat. No. 2,990,503 patent, by contrast, describes a two directional power flow system that utilizes multiple devices such as a vari-drive, directional clutches, synchronous motor/generator, etc. In this system, the synchronous machine is automatically controlled when in a driving mode of operation, such that the ratio change in the drive gears is adjusted in dependence upon the torque and power of the driving motor, as ascertained by the magnitude of the current drawn by the machine. The system operates to maintain the magnitude of the current at a predetermined level.
Finally, a vari-drive toroidal-type transmission is shown in the U.S. Pat. No. 2,153,252 patent, driving an alternating current motor. The transmission allows for the supplying of power at various selective amounts from a constant speed motor, and provides a means for varying the proportionate actual speeds of rotating field and armature members of an electric machine.
All of the aforementioned prior art patents are directed to power systems quite different from the differential drive system for frequency control of an induction generator, as described by the present invention. None of these patents disclose or suggest an electromechanical differential drive which controls the excitation frequency of an induction generator in such a way that the negative slip-frequency is controlled as a function of the input (engine) speed and the electric load on the generator while the generator speed and voltage are not controlled to a fixed per unit value.
A somewhat simple system which utilizes a synchronous generator as an exciter for an induction generator is disclosed in co-pending U.S. patent application Ser. No. 220,371, for "Induction Generator/Dual-Samarium-Cobalt Generator Combination", assigned to the assignee herein. The exciter generator and induction machine disclosed therein are driven through a fixed ratio gear system which causes the exciter generator to run at a slower speed relative to the induction machine. Under this arrangement, the induction machine operates as if it is in an overspeed condition and functions in a generator mode. While this relatively non-complex system is quite desirable for certain uses, it is somewhat limited in that it is insensitive to the different loads that may be imposed on the induction generator. By contrast, the present power system, while utilizing a highly desirable induction generator as a primary source of power, also provides the added feature of being able to adjust the slip frequency. This flexibility is particularly desirable when the power system is utilized in aircraft, where loads typically may vary a great deal. The ability to vary the slip frequency tends to prevent the induction machine from dissipating too much heat on the windings because of current flows higher than necessary.
The disclosures of the aforementioned prior art U.S. patents and copending patent application are hereby incorporated herein by reference.